Gastric bypass. Subjects will undergo surgery after Study Day 3. The same surgeon (J.C.E.) will perform all bypass procedures using standard techniques. Briefly, a small (~20 mL) proximal gastric pouch will be created by stapling across the stomach, and a 150 cm Roux-Y limb constructed by transecting the jejunum 30cm distal to the ligament of Treitz and creating a jejunojejunostomy 150 cm distal to the transection.